Why? Unlike Matrix it's actually an Internet Standard.
Should this be fixed (e.g. through regulation), XMPP is absolutely a sound choice - given, as you say, its 20 years of experience, evolution and deployment track record.
In theory, you could achieve with it what Matrix achieves, but it would probably be more feasible to start from scratch - like Matrix did
That said, I see how it currently has advantages when it comes to resource consumption as well as internal deployment to centralized services.
But if I would have to bet on a standard which could become it right now, then it would be Matrix.
It convinces when it comes to adoption, concept, features and growth.